Perhaps you should try some spelling variations:
observations theory » observations towards
operation theory » operations theory, iteration theory, generation theory
operator » operators
thereon » thereof, therein

You may be able to get more results by adjusting your search query.

• If you are trying to use Boolean operators, they must be ALL CAPS: (( operator theory ) OR ((( operation theory ) OR ((( observations theory ) OR ( observations thereon )))))).
• The words AND, OR and NOT may confuse the search; try adding quotes: "(( operator theory ) or ((( operation theory ) or ((( observations theory ) OR ( observations thereon ))))))".
• Adding a wildcard symbol may retrieve word variants: (( operator theory ) or ((( operation theory ) or ((( observations theory ) OR ( observations thereon ))))))*.